Such an EPR resonator ordinarily includes:
a cylindrical body which has an RF absorption of less than 5% at RFs below 1 kHz,
a first plunger delimiting the resonating volume within the cylindrical body in an axial direction at a first end and
a second plunger delimiting the resonating volume within the cylindrical body in an axial direction at a second end,
the second plunger having an opening for inserting an EPR sample into the EPR resonator.
A device of this type is known from U.S. Pat. No. 5,345,203 and from DE 41 25 655 C2.
Standard CW and Rapid Scan EPR spectroscopy methods, either of high or medium or low sensitivity type of implementations, require a simultaneous application of static, RF and microwave magnetic fields upon the material (i.e. the EPR sample) under study contained within a microwave cavity. Sensitivity in EPR spectroscopy is reflected by the signal to noise ratio of a measured EPR spectrum.
A microwave field, with frequency above 1 GHz, is called “internal” when its resonant energy is confined within the transmission line and the metallic walls of a microwave cavity, using one of its resonance modes. With respect to the EPR resonator design and technology the degree of this confinement draws its own borders between high sensitivity, medium or low sensitivity implementations.
To qualify for high sensitivity CW EPR applications (i.e. obtaining maximum EPR signal amplitude for a given unit of supplied microwave power) a microwave resonator may be designed for an optimum volume of an EPR sample (i.e. not too low sample volume—leading to low sensitivity by decreasing the number of EPR spins—and not too high sample volume—leading to medium or low sensitivity by decreasing the EPR signal through a reduced Q-factor of the EPR cavity). Furthermore the microwave cavity should have less than 40 dB loss of its resonant energy through various leakage mechanisms to the exterior of the cavity.
The RF field, with frequencies from kHz to MHz, is called “external” when it is generated by coils placed outside of the microwave cavity and when its energy is neither resonant to nor confined in the microwave cavity. Ideally the metallic walls of the microwave cavity should not interact with the external RF field. Unfortunately such interactions exist in the form of RF Eddy currents which directly influence several parameters of the external RF field across the EPR sample: amplitude (also known as the RF “transparency”), phase and homogeneity.
Furthermore, yet equally important for high-sensitivity EPR spectroscopy, the RF Eddy currents in the metallic walls of the EPR microwave cavity will produce also secondary effects, like Joule heating and vibrations of the microwave cavity, the latter caused by interactions between RF Eddy currents and the static magnetic field.
All these effects caused by presence of RF Eddy currents produce negative effects on the spectroscopic data of an EPR experiment.
The state-of-the-art simply relies on a local RF transparency for the cavity walls in nearest proximity to the EPR sample, while the entire issue of RF field homogeneity at the EPR sample location has not been seriously considered yet.
To date EPR microwave cavities for high-sensitivity applications consist of an assembly of sub-components, that are often electrically interconnected. These sub-components define a resonator volume surrounded by metallic walls, designed to be almost untransparent for the resonant microwave field. By selecting a specific microwave resonant mode with cylindrical symmetry (TE01n, where n is an integer and is chosen between 1 to 3) a high-sensitivity resonator can be designed with sub-components having cylindrical shapes: one cylindrical body called “body” (that defines the radius of cylindrical cavity) which is delimited at its axial ends by two plungers. Thereby the interior space between the plungers defines the cavity length. At least one plunger has an opening for bringing the sample or other EPR specific tools (e.g. dewars, sample holder) into the resonator volume.
Prior art high-sensitivity cylindrical CW EPR cavities propose two engineering solutions for the problem of RF Eddy currents:
a) the cylindrical body of the resonator is made of a wire wound conductive structure that is coated with a dielectric, where windings extend axially to form a solenoid, and
b) the cylindrical body of the resonator is made of a dielectric which is used as support for one or more thin metallization layers, sometimes accompanied by a slotting of the metallization layer.
In the above solutions only modifications of the EPR cavity body are addressed. Plungers delimiting the cavity at its axial ends usually are made of solid metal or of metallized dielectric (without any slotting). Prior art plungers, at least for cylindrical TE01n microwave resonance modes, are not optimized for the suppression of RF Eddy currents. Moreover low loss full metal transmission lines (waveguides or coaxial cables) that connect the EPR cavity to the microwave bridge, being placed sidewise to the cavity body may contribute to RF Eddy currents.
In conclusion prior art solutions succeed only to partially and locally mitigate the formation of RF Eddy currents, specifically in the cavity body walls. It is known that this partial approach might work well for obtaining sufficient RF transparency of the EPR cavity, at least around the region of interest (i.e. the EPR sample volume as placed in the center of cavity), as long as RF homogeneity, thermal loading of cavity and the mechanical oscillations are not an issue. Prior art solutions are not sufficient if e.g. the source of the external RF field has similar or bigger size compared to the EPR cavity as required for bigger EPR samples and, if RF field homogeneity in the bigger EPR sample volume has to be kept. In addition the power of the RF field and microwave field is increased for high sensitivity EPR. Therefore the thermal loading of the cavity and/or the oscillations produced by interaction of RF Eddy currents with the strong static magnetic field become important parameters.
U.S. Pat. No. 5,345,203 and DE 41 25 655 C2, cited above, disclose a “Resonator arrangement for electron spin resonator spectroscopy”—contains cylindrical TE012 EPR resonator that has body with wire wound (solenoid) design. Here, as mentioned above:
a) the two axial plungers—top and bottom—used for closing the microwave TE012 cylindrical cavity and for defining its axial length, are made of solid metal.
b) the antenna used for microwave coupling is placed sideways and perpendicular to the resonator axis, therefore a hole in the wire wound solenoid body is needed. Further, the coaxial cable is soldered in this hole, forming, as side effect, an electrical connection between a couple of solenoid turns in the EPR cavity wall. This microwave coupling arrangement forces some winding turns of the solenoid body (cavity wall) to become electrically closed and therefore prone to develop RF eddy currents. In this configuration the electrical connection of the shield of the coaxial cable with the EPR cavity body creates closed loops and thereby significant contribution to RF Eddy currents as well.
In particular, U.S. Pat. No. 5,345,203 and DE 41 25 655 C2 show in FIG. 6 a cylindrical wire wound body, a first plunger and a second plunger, both of which are made entirely of metal. As the plungers are made of metal they are not RF transparent. As the body is wire wound an external RF field is capable of traversing the EPR sample volume, locally in the center of microwave cavity.